This invention relates to change speed gear trains and to drive transmissions incorporating such gear trains, and to highway vehicles incorporating such drive transmissions.
The expression "highway vehicles" as used in this specification and claims, means a vehicle, such as a city bus or coach for transporting passengers, and/or a chassis for such a vehicle, which is intended for use primarily on highways or roads and which has a gearbox providing at least three forward transmission ratios, and which is capable of sustained operation at the usual operating speeds of highway vehicles. The expression is intended to cover general purpose vehicles and vehicle chassis which fall within the above definition, but which by virtue of, for example, a four wheel drive, are also capable of operation over rough terrain.
Highway vehicles usually are provided with only one transmission ratio for travel in the reverse direction, and the present invention is not relevant to shuttle-type vehicles such as crawler dozers and tractor-type front end loaders and the like which occasionally travel on highways to and from their place of work. Such shuttle-type vehicles are often provided with transmissions having multiple forward and multiple reverse transmission ratios, and the design criteria applicable to such vehicles are completely different from those applicable to highway vehicles with which this invention is concerned. This is due to the total difference between these two types of vehicles so far as concerns operating conditions, and cost and space considerations, and passenger and safety factors, and servicing criteria.
More particularly, but not exclusively, this invention relates to city buses or coaches. By the expression "city bus" as used in this specification and claims is meant a passenger transport vehicle capable of carrying at least fifteen passengers.
The drive transmissions of many public service vehicles such as city buses currently in use comprise planetary or epicyclic gearboxes. Such gearboxes are complex, expensive and difficult to understand and service, but despite these disadvantages have been developed to a stage at which they provide smooth operation with automatic ratio changing facilities, and are now universally accepted and used.
Constant mesh multiple layshaft type gear trains are considerably simpler than planetary gear trains and many proposals have been made for their use in various types of machines including some proposals for their use in certain land vehicles. However, such gear trains have not been widely accepted for use in such vehicles, and in public service vehicles, such as buses, the planetary gear train is still universally used. Among the reasons for this limitation in the use of multiple layshaft gear trains is the fact that change speed gear trains for use in highway vehicles are subject to a demanding series of inter-related technical requirements which have hitherto remained inadequately satisfied. These requirements include the following:
Firstly, there is the question of size, complexity and cost. Many previous proposals for multiple layshaft gear trains have required the use of a gearbox having a size quite unacceptable for incorporation in the crampled engine compartment of modern highway vehicles. Moreover, even where a gearbox of modest proportions has been proposed, it has usually been the case that the layout of the gear train has been such that the provision of the necessary range of transmission ratios needed for highway vehicle use (for a bus this range is from about 4 to 1 for bottom gear to 1 to 1 for top gear) has not been possible. In such circumstances, modification of the gear train to achieve the necessary ratios would in most cases result in a significant increase in the size of the gearbox.
Secondly, there is the question of servicing and access to the wearing components of the gearbox. Previous proposals for multi-layshaft constant mesh gear trains have not provided sufficient attnetion to the need for adequate clutch service life in combination with easy access to the clutches for servicing purposes, without introducing unacceptable drag into the drive transmission. Previous proposals have in many cases taught the use of unlubricated external clutches, which would have quite unacceptable service life for highway vehicle use. Where internal clutches have been proposed, these have usually been subject to uncontrolled splash lubrication (whereby uncontrolled and unacceptably high drag results) and/or the clutches have been located so as to be relatively inaccessible for servicing purposes.
French Pat. No. 1,043,561 discloses a two-layshaft constant mesh gear train providing five transmission ratios (including a direct-drive top gear) by means of four dry external cone clutches positioned one at each end of each of the layshafts so as to achieve double usage of both of the layshafts. The arrangement of this patent, i.e., unlubricated external clutches and double usage of both layshafts, is subject to the significant disadvantages of inadequate clutch service life and inaccessibility of two of the clutches when the gear train is mounted adjacent a prime mover.
U.S. Pat. No. 3,064,488 likewise discloses a multi-layshaft constant mesh gear train with multiple intermeshing layshaft gears providing a large number of transmission ratios and complex torque input paths with multiple torque-transmitting mesh points. The teaching of this prior patent is subject to the above-mentioned disadvantages of unacceptable size and complexity. Moreover, the teaching is once again of double usage of each layshaft with half of the clutches relatively inaccessible at the wrong end of the gear train.
An object of the present invention is to provide a constant mesh change speed gear train offering improvements in relation to at least some of the requirements and technical problems identified above.